A communication system is formed, at a minimum, of a transmitter and a receiver connected by way of a communication channel. Information-containing, communication signals generated by the transmitter are transmitted upon the communication channel to be received by the receiver. The receiver recovers the informational content of the communication signal.
A wireless, or radio, communication system is a type of communication system in which the communication channel is a radio frequency channel defined upon the electromagnetic frequency spectrum. A cellular communication system is exemplary of a wireless communication system.
The communication signal transmitted upon the radio frequency channel is formed by combining, i.e., modulating, a carrier wave together with the information which is to be transmitted. The receiver recovers the information by performing a reverse process, i.e., demodulating, the communication signal to recover the information.
When the communication signal transmitted by the transmitter is received at the receiver, the communication signal must be of at least a minimum energy level and signal quality level to permit the receiver to recover the informational content of the transmitted signal.
Several other factors affect the recovery of the informational content of the transmitted signal.
The signal transmitted upon the communication channel to the receiver is susceptible to, for instance, reflection. Signal reflection of the transmitted signal causes the signal actually received by the receiver to be the summation of signal components transmitted by the transmitter by way of, in some instances, many different paths, in addition to, or instead of, a direct, line-of-sight path. As the distance separating the transmitter and receiver increases, however, the reflected signal components become increasingly less significant than signal components transmitted upon direct, or nearly-direct, paths. As the distance separating the transmitter and receiver increases, therefore, a highly-directional antenna is best able to detect signals transmitted by a transmitter. Because reflected signal components form relatively insignificant portions of the signal received by the receiver at such increased separation distances, a directional antenna directed towards the transmitter detects significant portions of the signal while also maximizing the coverage area of the receiver. A nondirectional antenna, capable of detecting greater levels of reflected signal components, is not required.
A signal simultaneously-transmitted by another transmitter upon the same, or similar, communication channel can interfere with the signal desired to be transmitted to a receiver. The signal transmitted to the receiver is therefore also susceptible to interference caused by such a simultaneously-transmitted signal. Co-channel and adjacent-channel interference are exemplary of types of interference to which the signal transmitted to the receiver might be susceptible.
As noted previously, when the distance separating the transmitter and receiver is relatively significant, a line-of-sight signal component becomes increasingly stronger vis-a-vis reflected signal components. And, at increased separation distances, reflected signal components form only a negligible amount of the power of the signal received by the receiver.
A directional antenna is best able to recover the informational content of a transmitted signal when the signal received at the receiver does not include significant levels of multipath signal components. Additionally, when the directional antenna includes nulls encompassing the locations from which interfering signals are transmitted, the interference caused by such interfering signals can be best minimized.
As mentioned previously, a cellular communication system is a wireless communication system. A cellular communication system includes a plurality of spaced-apart, fixed-site transceivers, referred to as base stations, positioned throughout a geographic area. Each of the base stations supplies a portion, referred to as a cell, of the geographic area. A moveably positionable, or otherwise mobile, transceiver, referred to as a mobile unit, can be positioned at any location (i.e., within any cell) within the geographic area encompassed by the cellular communication system. The mobile unit, when so-positioned, can transmit communication signals to at least one of the base stations.
As the mobile unit moves between cells, the mobile unit is "handed-off" from one base station to another base station. That is to say, when a mobile unit in communication with a first base station travels out of the cell defined by the first base station and into the cell defined by a second base station, the mobile unit commences communication with the second base station. The hand-off from the first base station to the second base station occurs automatically and without apparent interruption in communication by one communicating by way of the cellular communication system.
Typically, the base stations of the cellular communication system each include an antenna device for transmitting signals to, and receiving signals from, mobile stations located anywhere within the cell. The signal actually received by the base station is sometimes a complex interference pattern formed of various reflections of the transmitted signals transmitted from the mobile by way of many various paths of a multipath channel and also of interfering signal components generated by other mobile units. The other mobile units may, for example, be in communication with another base station or be transmitting signals on an adjacent communication channel.
For the same reasons as those described above with respect to a generic transmitter and receiver, as the distance separating the mobile unit and a base station increases, the power of the multipath components tend to become progressively weaker relative to a signal transmitted upon a direct path between the mobile unit and the base station. A directional antenna is best able to receive such a signal and is also capable of maximizing the range of operability of the base station to send and to receive signals. To minimize the effects of interference caused by the transmission of signals generated by other mobile units, nulls forming a portion of the antenna beam configuration located at the position of the other mobile units can best minimize the adverse effects of such interfering signals.
As utilization of cellular communication networks, as well as other types of wireless communication systems, become increasingly popular, it has become increasingly necessary to efficiently utilize the radio frequency channels allocated for such communication. In the example of a cellular communication system, a base station having an antenna apparatus exhibiting increased carrier-to-noise and carrier-to-interference ratios would facilitate efficient utilization of the allocated frequency channels. Other types of wireless communication systems would similarly benefit from the utilization of such an antenna.
It is in light of this background information related to wireless communication systems, such as a cellular communication system, that the significant improvements of the present invention have evolved.